Thus the possible choices for \(|\bar{u}_fMu_i|^2\)  are restricted. So, for all of them \(|\bar{u}_fMu_i|^2\)  can be calculated and a universal expression can be derived. This expression can be used for all possible interaction matrices. Such an expression was derived in [1]} but \(|\bar{u}_fMu_i|^2\)  is expressed through the three dimensional quantities in laboratory reference frame. In most cases it is preferable to have Lorentz's covariant expression which is derived below.
